The present invention relates to the magnetic resonance art. It finds particular application in conjunction with self-shielded gradient and RF coil assemblies for magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it will be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other applications which require RF fields and gradient magnetic fields.
Heretofore, magnetic resonance imagers have utilized a cylindrical superconducting magnet assembly which generates a temporally constant primary magnetic field axially through a room temperature bore or central cylinder of the magnet assembly. The magnet windings are maintained at liquid helium temperatures in a cryostat. This structure is double walled to provide a vacuum space, intermediate cold shields, and room for superinsulation. Cryostat construction is optimized to minimize helium boil-off. The room temperature bore of the magnet provides space for gradient magnetic field coils, RF coils, and, of course, the object or person to be imaged.
The gradient coils generate axial fields which have gradients in three mutually orthogonal directions, x, y, and the axial direction z. Current waveforms are applied to the gradients during the MR process. The gradients in turn induce eddy currents in the metal structures of the magnet cryostat. The magnetic fields of the eddy currents then become part of the magnetic fields seen by the object being imaged. To the extent that the eddy currents mimic the gradient that caused them, they could be corrected by pre-emphasis. In general, it helps to maintain some spacing between the gradient and magnet structures that might sustain eddy currents. More recently, self shielded gradients have been developed that eliminate the eddy currents in the magnet structure.
Self shielded gradients consist of a primary set of coils on a cylindrical former and a shield set of coils on a second former at a larger radius. At radii outside the shielded gradient, the magnetic fields produced by the primary coils are substantially cancelled by those of the shielding coils. The gradient fields of both sets of coils combine to produce axial directed fields with substantially first order intensity gradients in the x, y, and z-directions. It is desirable that the primary and shield coils have a radius ratio of 1.3 or more so the shield coil does not cancel too much of the field of the primary within the imaging volume. With self shielded gradient coils, the distance from the primary gradient coil to the magnet structures is not critical.
The overall efficiency or power dissipated by the gradient coil is strongly dependent on the spacing between the primary gradient coil and the shield gradient coil. Efficiency is optimized, i.e. power consumption is minimized, when the diameter of the primary gradient field coil is minimized and the diameter of the shield coil is maximized. However, 100 cm. bore magnets in general cost more than smaller, e.g. 90 cm. bore magnets because of materials costs. In order to reduce magnet cost, one would like to reduce the bore diameter consistent with homogeneity requirements. Likewise, in order to improve the gradient efficiency and get more gradient per imposed ampere, one would like to use the maximum space available for the outer coils of a shielded gradient. The maximum diameter of the shield gradient coil assembly is limited by the interior bore of the superconducting magnet or, more commonly, by a shimset of ferrous metal shims mounted within the bore for improving the homogeneity of the main magnetic field.
An RF body coil which is inserted inside the gradient coil generally had an inner diameter of 55-60 cm. RF screens are placed at some radius beyond the RF coil with the specific purpose of generating a known eddy current pattern that is not RF lossy. In addition, there is a degree of isolation between the RF and the gradient coils. For example, the RF screen has been applied to the inner surface of the gradient coil former. The screen is made of overlapping strips of adhesive backed copper about 2-3 inches wide completely covering the surface. The overlap region serves to capacitively couple the strips at 21 to 64 MHz so they look like a reasonably conductive surface. If the adhesive on the copper is RF lossy, it will reduce the Q-factor of the RF coil and thus the signal to noise ratio for NMR. The copper is in strips to reduce the gradient eddy currents generated therein.
One of the important design considerations of the RF screen is the distance between the RF coil conductive elements and the screen. Eddy currents in the screen also generate fields that oppose the fields of the RF coil reducing the overall field. In a sense these "image currents" mimic, in a functional sense, the shield coils of the gradient coil. The screen must produce predictable, reproducible results without RF losses. Beyond the RF screen there is substantially zero RF field.
One disadvantage of the use of adhesive backed copper in the RF coil shield is that it is labor-intensive. Typically, on the order of 32 or more segments are required around the periphery of the inner diameter of the gradient coil bore. A further disadvantage is that the adhesive can be RF lossy and lower the achievable signal-to-noise ratio.
Another method for constructing the RF shield is to use a printed circuit approach with a low-loss substrate.
One disadvantage of the printed circuit board approach is its cost. Substrate materials have been used to obtain the desired low loss characteristics and provide a means for mounting the screen to the gradient inner diameter. This increases processing and fabrication costs as well.
A third and more cost-effective method is to use a fine copper mesh, much like window screen. The broken pattern of the woven mesh is intrinsically worse at conducting low frequency eddy currents than solid copper.
One disadvantage to the use of RF mesh is that mounting the mesh to the interior diameter of the gradient coil is difficult. The screen must be mounted securely with good dimensional control. Generally, an additional substrate is required to support the mesh. This increases the manufacturing costs and consumes valuable space.
Thus, the prior art shielded RF and gradient coils assemblies are thicker in radial dimension. Such thicker RF and gradient coils can be utilized and still maintain the conventional inner diameter of the bore if large diameter, e.g. 100 cm. bore superconducting magnets are utilized. However, the cost of the superconducting magnet varies with its diameter. Accordingly, it is generally considered desirable to reduce the diameter of the inner bore of the magnet, not enlarge it.
Reducing the spacing between the primary and shield gradient coils would reduce efficiency and create significant heating problems. That is, much of the energy consumed by the gradient coil is turned into heat. In an inefficient system in which a large amount of heat is generated, this creates problems in dissipating the heat. Similarly, even a small increase in the relative diameters of the primary RF and shield coils can have a significant change in the amount of energy consumed. For example, a 2% increase in the diameter of the gradient shield coil can result in a 20% decrease in heat energy dissipation.
The present invention provides a new and improved shielded RF and gradient coil design which overcomes the above-referenced problems and others.